In human sexual behavior, promiscuity denotes sex with relatively many partners. In polygamy it is distinguished from promiscuity.

Promiscuity is common in many animal species. Some species have promiscuous mating systems, ranging from polyandry and polygyny to mating systems with no stable relationships where mating between two individuals is a one-time event. Many species form stable pair bonds but still mate with other individuals outside the pair. In biology, incidents of promiscuity in species that form pair bonds are usually called extra-pair copulations.

Human promiscuity

What sexual behavior is considered socially acceptable, and what behavior is "promiscuous", varies much among different cultures, and within a culture different standards are often applied to people of different gender and civil status. In many cultures, while male promiscuity previously had glamorous connotations that acted as an affirmation of masculinity, female promiscuity was seen as a sign of emotional instability and loose morals in women.

Accurately assessing people's sexual behavior is difficult, since there are strong social and personal motivations, depending on social sanctions and taboos, for either minimizing or exaggerating reported sexual activity. Extensive research has produced mathematical models of sexual behavior comparing the results generated with the observed prevalence of sexually transmitted illnesses (STIs) to statistically estimate the probable sexual behavior of the studied population.

The number of sexual partners an individual has varies within a lifetime, and varies widely within a population. In the U.S., a 2007 national survey had the following results: the median number of lifetime female sexual partners reported by men was seven; the median number of male partners reported by women was four. It is possible that men exaggerated their reported number of partners, women reported a number lower than the actual number, and/or a minority of women had a sufficiently larger number than most other women to create a mean significantly higher than the median. Twenty-nine percent of men and nine percent of women reported to have had more than 15 sexual partners in their lifetimes. Studies of the spread of STIs consistently demonstrate that a small percentage of the studied population have more partners than the average man or woman, and a smaller number of people have fewer than the statistical average. An important question in the epidemiology of sexually transmitted infections is whether or not these groups copulate mostly at random (with sexual partners from throughout a population) or within their social groups (assortative mixing).

A 2006 comprehensive global study (analyzing data from 59 countries worldwide) found no firm link between promiscuity and STIs, with poverty and mobility being more important factors. This contradicts other studies.

Researchers said Britain's position on the international index "may be linked to increasing social acceptance of promiscuity among women as well as men". Britain’s ranking was "ascribed to factors such as the decline of religious scruples about extramarital sex, the growth of equal pay and equal rights for women and a highly sexualised popular culture". (telegraph.co.uk)

Another study that looked at promiscuity based on total number of sex partners found that Austrian men were the most promiscuous males in the world with 29.3 sexual partners on average, while New Zealand women were the most promiscuous females in the world with an average of 20.4 sexual partners. New Zealand was the only country studied where women averaged a higher number of sex partners than men.

One study found that people from developed Western countries were more promiscuous than people from developing countries in general, while the rate of STIs was higher in developing countries.

Male promiscuity

A 1994 study in the United States looking at the number of sexual partners in a lifetime found that 20% of heterosexual men had 1, 55% had 2-20, and 25% had 20 or more. Early studies found men with homosexual contact were more likely to have a very large number of sexual partners, but a 1989 study found a very high number of partners (over 100) to be present but rare in that demographic. The difference was attributed to sampling problems with earlier studies, and the influence of AIDS.

Female promiscuity

A 1994 study in the United States found that almost all married (heterosexual) women reported having sexual contact only with their husbands, and unmarried women almost always reported having no more than one sexual partner in the past three months. Lesbians who had a long-term partner reported having fewer outside partners than heterosexual women.

Since at least 1450, the word "slut" has been used, usually pejoratively, to describe a sexually promiscuous woman. In and before the Elizabethan and Jacobean eras, terms like "strumpet" and "whore" were used to describe women deemed promiscuous, as seen for example in John Webster's 1612 play The White Devil.

Primitive promiscuity

Extra-pair copulation in animals

In the animal world, some species of animals, including birds such as swans, once believed monogamous, are now known to engage in extra-pair copulations. Although social monogamy occurs in about 90 percent of avian species and about 3 percent of mammalian species, investigators estimate that 90 percent of socially monogamous species exhibit individual promiscuity in the form of extra-pair copulations.

Two examples of promiscuous animals are the primates chimpanzees and bonobos. These species live in social groups consisting of several males and several females. Each male copulates with many females, and vice-versa. In bonobos, the amount of promiscuity is particularly striking because bonobos use sex to alleviate social conflict as well as to reproduce.